I. Field of the Invention
The present invention relates generally to crankshafts and, more particularly, to a multi-piece crankshaft and method for constructing the same.
II. Description of Related Art
Many previously known crankshafts are made from a heavy single body of cast metal which is then machined so that the main shaft, crankpins and counterweight are all of a one-piece construction. Machining such crankshafts, however, requires specialized equipment and is relatively expensive in manufacturing cost.
There are, however, previously known crankshafts which are constructed from multiple pieces for low cost construction. Such crankshafts are oftentimes used in small two-cycle engines although they can also be used in other types of engines or compressors.
These crankshafts that are used in small two-cycle engines typically comprise a main shaft having one end connected to the counterweight. One end of the crankpin is then also connected to the counterweight at a position radially spaced from the axis of the main shaft. Additionally, a flywheel support taper is formed adjacent the other end of the main shaft. This flywheel support portion includes a tapered mating surface for mating with the flywheel. Additionally, an axially or longitudinally extending key slot is formed in the flywheel taper support for locking the flywheel and main shaft together by a key.
Typically in these multi-piece crankshafts, the counterweights are constructed from an inexpensive material, such as powdered metal or steel stampings, and the counterweight has holes formed in it corresponding to the position of the crankpin and main shaft. The crankpin and main shaft are then constructed from conventional high strength round stock. One end of the main shaft as well as one end of the crankpin are then knurled, splined or otherwise deformed and pressed into the openings formed in the counterweight to thereby form the crankshaft.
In order to form the flywheel support taper, the main shaft is typically machined using lathes or screw machine equipment to form the taper on the main shaft. A second machining operation is then needed to form the axially extending key slot in the flywheel taper support.
A primary disadvantage of these previously known multi-piece crankshafts is that the main shaft and crankpin must be parallel to each other within very high tolerances. Similarly, the precise position of the flywheel support taper as well as the angular and axial position of the keyway must be maintained within very small tolerances. Similarly, both the width and depth of the keyway must be manufactured within very small tolerances in order to meet modem manufacturing specifications.
However, during the pressing operation of the main shaft and crankpin into the counterweight, the main shaft and crankpins often become skewed relative to each other and the resulting crankpin assembly must be either corrected by bending the crankpin and main shaft relative to each other or, in some cases, discarded as scrap. Furthermore, in many cases the precise position of the flywheel support taper together with its key slot may not meet manufacturing tolerances where, during the pressing operation of the main shaft into the counterweight, axial movement of the counterweight relative to the main shaft occurs from spring back or the like.